Australian fires have generated huge swathes of phytoplankton in the Southern Ocean

Large fires not only wreak havoc on land, they also produce unexpected effects even in the ocean. Thus the aerosols emitted by the gigantic bush fires which devastated millions of hectares and caused the death of more than a billion animals, from December 2019 to March 2020 in Australia, also caused a massive proliferation of microalgae. thousands of kilometers further south in the Southern Ocean. This is what analyzes a study published Wednesday, September 15 in the journal Nature.

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The ash resulting from the burning of forests and bushes contains particles of iron, nitrogen, phosphorus. Carried by high winds, those of the Australian disaster – some of which were projected up to 16 kilometers high – went to fertilize vast expanses of the Southern Ocean. These cold waters, usually poor in iron and not very productive in phytoplankton, then experienced blooms. ” unprecedented “, according to the authors of this study. Satellite images showed the green color that appeared on the surface as an anomaly of a magnitude that surprised scientists.

“It was so extraordinary, so dense: in two regions in particular, the chlorophyll concentration had more than doubled compared to the average of the data for the previous twenty-two years, reports one of the study’s co-authors, Joan Llort, a researcher at the University of Tasmania and the Barcelona High Performance Computing Center. Imagine this desert turning into a meadow for four months! “

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In real size

It was also so unusual in this part of the world that the international team of scientists wanted to verify that it was not a misinterpretation of the signal detected during readings of satellite images.. Its members therefore worked both with aerial observations, the analysis of the density of aerosols present in the atmosphere from the data recorded by the European program Copernicus, the modeling of the trajectory of the plumes of particles, the analysis Argo float readings and sampling taken over Tasmania during one of the ash passages from Australia.

The impact of iron on distant ecosystems has already been addressed by modeling, but in the winter of 2019-2020, the phenomenon was observed in real life. Why is it so appealing to scientists? On Earth, half of photosynthesis takes place in the ocean: the planet’s oxygen therefore depends largely on algae and other marine microorganisms. This primary production participates in the complex mechanisms of interaction between the ocean and the atmosphere. In other words, it influences the climate.

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